[Paper Review] Electron Crescent Distributions as a Manifestation of Diamagnetic Drift in an Electron‐Scale Current Sheet: Magnetospheric Multiscale Observations Using New 7.5 ms Fast Plasma Investigation Moments

A. C. Rager, J. Dorelli+17 more|arXiv (Cornell University)|Jun 26, 2017

Ionosphere and magnetosphere dynamics19 references62 citations

TL;DR

This study uses 7.5 ms electron moment data from the MMS Fast Plasma Investigation to demonstrate that electron crescent distributions in an electron-scale current sheet are primarily caused by diamagnetic drift due to pressure gradients, not meandering orbits. The results show that deviations from E×B drift are explained by electron diamagnetic drift, and the electron momentum equation is violated at 7.5 ms resolution during large electric field fluctuations, suggesting complex microphysics beyond standard fluid models.

ABSTRACT

We report Magnetospheric Multiscale observations of electron pressure gradient electric fields near a magnetic reconnection diffusion region using a new technique for extracting 7.5 ms electron moments from the Fast Plasma Investigation. We find that the deviation of the perpendicular electron bulk velocity from $E imes B$ drift in the interval where the out-of-plane current density is increasing can be explained by the diamagnetic drift. In the interval where the out-of-plane current is transitioning to in-plane current, the electron momentum equation is not satisfied at 7.5 ms resolution.

Motivation & Objective

• To determine the physical origin of electron crescent distributions observed in electron-scale current sheets near magnetic reconnection.
• To test whether non-gyrotropic electron distributions are driven by diamagnetic drift rather than meandering orbits.
• To validate a new 7.5 ms electron moment extraction technique for resolving sub-ion scale plasma dynamics.
• To assess the validity of the electron momentum equation at high temporal resolution in reconnection regions.

Proposed method

• A new algorithm was developed to extract 7.5 ms electron moments from raw MMS Fast Plasma Investigation (FPI) data, improving temporal resolution over the standard 30 ms product.
• The electron momentum equation was analyzed in the frame of the guiding center drift, separating gyrotropic and non-gyrotropic pressure tensor components.
• The electron pressure tensor was decomposed into perpendicular and parallel components to isolate the contribution of diamagnetic drift.
• Time-resolved comparisons were made between E×B drift, observed bulk velocity, and pressure gradients to identify deviations explained by diamagnetic drift.
• The divergence of the non-gyrotropic pressure tensor was evaluated to rule out its role in momentum imbalance.
• High-resolution data were used to assess discrepancies in the electron momentum equation during intervals of large electric field fluctuations.

Experimental results

Research questions

• RQ1 What causes the observed electron crescent distributions in the electron-scale current sheet, and is it due to diamagnetic drift or meandering orbits?
• RQ2 Can 7.5 ms electron moments resolve microphysical processes that are averaged out at 30 ms resolution?
• RQ3 To what extent does the electron momentum equation hold at 7.5 ms resolution in the reconnection diffusion region?
• RQ4 Is the deviation of electron bulk velocity from E×B drift explained by electron diamagnetic drift due to pressure gradients?
• RQ5 What is the role of large-amplitude electric field fluctuations in violating the electron momentum equation at high time resolution?

Key findings

• The deviation of the electron perpendicular bulk velocity from E×B drift between 13:07:01.199 UT and 13:07:02.180 UT is primarily explained by electron diamagnetic drift due to perpendicular pressure gradients.
• The 7.5 ms electron moment technique successfully recovers finer temporal structure in the perpendicular bulk velocity, improving agreement with E×B drift and validating the algorithm.
• The electron momentum equation is not satisfied at 7.5 ms resolution during the interval of large electric field fluctuations (starting at 13:07:02.2 UT), indicating unresolved microphysics.
• The non-gyrotropic component of the pressure tensor does not explain the momentum imbalance, ruling out its direct role in the discrepancy.
• The observed crescent distributions precede the onset of large electric field fluctuations, suggesting they are a signature of diamagnetic drift rather than energy dissipation.
• The results are consistent with the absence of significant magnetic energy dissipation (J·E′ ≈ 0) during the crescent phase, challenging the assumption that crescents imply active reconnection.